Semichemical pulping process for soft woods



Dec. 18, 1962 R. H. RAscH ET AL SEMICHEMICAL PULPING PROCESS FOR soFTwooDs 2 Sheets-Sheet l Original Filed Sept. 19, 1958 Dec. 18, 1962 R. H.RAscH ET AL SEMIOHEMIOAL PULPING PROCESS FOR SOFT WOODS 2 Sheets-Sheet 2Original Filed Sept. 19, 1958 United States Patent Oflce 3.069 31)SEMECHEMICAL PULPING PRQCESEi FR SQFT lil/@GDS Royal H. Rasch and Paulis. ileemhuis,

Sutton, Waterford, Pa., assignors to Hammermill Paper Company, Erie,Pa., a corporation of Pennsylvania Continuation of application Sept. i9,i953, Ser. No. 762,-

@35. This appiication et. i4, Iliiil, Ser. No. 64,666

itil Claims. (Cl. l62--86) This invention relates to an improved processfor the production of high quality bleached pulp Ifrom coniferous woods,commonly referred to as softwoods. The pulps produced with the presentinvention conform with the specifications of those required for theproduction of fine papers, as that term is most generally used in theindustry. The present application is a continuation of Rasch et al.application, Serial No. 762,035, tiled on September 19, i958, nowabandoned.

A primary urpose of the invention has been to provide a process of thecharacter indicated which is readily adapted for the production of avariety of different types of pulp, i.e. pulps which dier in certain oftheir important ci aracteristics so that the same equipment may be usedunder readily variable conditions to produce, at will, pulps havingcharacteristics desirable for a number of different purposes. Theinvention is particularly concerned with the production of line papersfor printing, writing and cover paper purposes, as distinguished fromwrapping and other coarse papers.

Fine papers of the variety of types now available are required to havedifferent characteristics of strength, opacity, brightness, and thelike, depending upon their intended use. The present invention isconcerned with a process, utilizing the same equipment which may be usedunder readily variable conditions as to temperature, concentration ofthe digesting liquor, duration or extent of cook, and the extent andcharacter of the subsequent chemical and mechanical treatment of theresulting pulp, to bring about the production of a variety of differenttypes of pulp which may be used alone or in conjunction with other pulpsproduced by the sameequipment for the formation of ne papers f differenttypes. An important consideration has been the development of such auniversal process which may be carried out on an economic basis. Thisrequires the ability to produce an adequate amount of pulp per daywithout the necessity of an excessive amount of equipment and withoutexcessive costs for chemicals, power and the like.

Other obiects, purposes and advantages of the present invention willappear from the following detailed description of various illustrativeembodiments of the same. Such detailed description of certainillustrative embodiments of the invention will be given hereinafter inrelation to the accompanying drawings which illustrate schematically thepreferred conduct of the process and certain permissible variationsthereof. ln the drawings:

FIG. l illustrates, by way of a iiow diagram, how the original woodchips are digested and otherwise treated for the production of thedesired unbleached pulp, and

PEG. 2 is a iow diagram showing the steps involved in the finaltreatment of the unbleached pulp to achieve the purposes of theinvention.

The properties of fine papers vary widely according to type and grade.Specications for these papers are increasingly being met by the use ofblends of several kinds of pulps. This trend is not entirely dictated byconsiderations of cost. In the paperma ing art a desired balance ofpaper characteristics frequently can be obtained better with a varietyof pulps rather than with a single type of liber.

Bleached deciduous (hardwood) soda pulps, and, more recently, deciduoussuilite pulps have been included in the furnishes of certain tinepapers, notably certain book and Erie, and Harvey 3,069,310 PatentedBec. 18, 1962 offset printing papers. These fibers contribute desirablyto a uniform sheet formation and a level printing surface, but they aredeficient in strength properties. Presently, bleached deciduous kraftand special quality deciduous semi-chemical pulps are finding acceptancein a number of fine paper mills. These newer libe'rs have relativelygood strength properties, are relatively low in cost, yet retain certainof the intrinsic merits of the shorter-bered pulps.

Despite the increasing utilization of the high quality deciduous bersreferred to, a usually larger component in the furnish of various typesand grades of line papers consists of one or more coniferous (softwood)pulps. The coniferous fibers are coarser and longer than deciduousfibers, averaging about 2.5 to 5.0 mm. in length as against 1.0 mm. Thelong-fibered stock provides certain desirable features. For example, itcontributes relatively high wet Web strength to a sheet and thus aids inrunning on the paper machine. Suitable coniferous pulps contributedesirable properties, some measurable and some intangile, which can besupplied only by the greater length of their component fibers.

The requirements for long-libered pulp are substantially being met atpresent by two types of wood cellulose iiber; bleached conifere-usconventional acid sulte pulp and bleached coniferous kraft pulp. In theearly history of paperniaking, cotton and linen rags provided the rawmaterial for tine papers but the demands of the tine paper industry havelong since greatly exceeded 4the available supply of rags. Later,bleached sultite pulp supplied a large proportion of the requirements inthe field. In more recent years, with more advanced techniquesl in thebleaching of coniferous kraft pulps, this liber has been usedincreasingly for high quality papers, particularly where strength is animportant consideration.

Tearing strength is one of the most important characteristics of certainhigh quality papers, such as certain grades of bond papers. Tearingstrength is the resistance, or drag offered by a sheet of paper when itis torn. This quality may be measured with precision for speciiicationpurposes. Also it may be qualitatively estimated by the consumer withouttesting equipment. Thus tearing a sheet becomes one of the simplesttests for characterizing an important toughness quality of a paper..Bleached coniferous kraft pulps are much superior in tearing strength toconiferous sultite pulps or to deciduous pulps as hereto-fore produced.Paper manufacturers have therefore frequently relied upon coniferouskraft pulps to meet exacting high tearing strength specilications and tomake up for strength deficiencies of other fibers necessarily present inthe furnish to supply other equally important properties.

On the other hand, in some line papers the toughness conferred by kraftliber is less important than certain other properties, such astranslucency, which can more readily be obtained with bleachedconiferous sultite pulp.

The manufacturer of a variety of types and grades of tine papers mustmeet many different quality specifications. It is frequently impossibleto do this with a single kind of pulp. For best results it is necessaryto have available a number of dierent fibers each with certain uniquecharacteristics or advantages. For example these pulps might include thefollowing:

(l) Coniferous sulfite pulplong fibered, easy beating, good webstrength, fair formation, moderate dry strength, relatively low opacity.

(2) Deciduous semichemical pulp-short ibered, easy beating, relativelypoor wet web strength, good formation, moderate dry strength, goodopacity.

(3) Coniferous kraftlong iibered and coarse, very hard beating, good wetweb strength, relatively poor formation, excellent dry strength, fairopacity.

From the standpoint of quality, exibility of operation, and cost, it isdesirable for a tine paper mill to:

(l) Employ a sucient variety of papermaking pulps, of the character ofthose listed above, in order to meet readily any desired combination ofspecifications.

(v2) Suppply all pulp requirements within their own pulp makingfacilities. This is not only economically advantageous, in that itavoids the necessity of purchasing market pulp, but it further avoidsthe considerable expense of handling and reconstituting the dried marketpulp. Moreover, quality advantage results from the ability of usingslush pulp and thus avoiding the sacrifice of certain pulp propertieswhich accompanies the drying of the market pulp for ship-ment. Inaddition the quality of the pulp is within the view and control of themill which is to utilize it. v

(3) Provide the necessary liber varieties in controllable ratios withoutrequiring a multiplicity of widely different pulping operations.

Heretofore it has beenk seldom that the advantageous circumstancesoutlined above could be met totally by a ne paper mill. Usually one ormore of the different pulps has had to be purchased. For example,although kraft pulp has heretofore been regarded as essential in thefurnish of many papers, the quantities of this fiber required by manypaper mills would not justify the economically minimum size kraft millwhich is about G tons per day.

In accordance with the present invention an improved process,eliminating the-problems of the kraft process,rhas been provided for theproduction of a wood cellulose fiber having a unique combination of theproperties particularly desirable'in the manufacture of` certain highquality ne papers. These are properties found in certain premirnurnbleached coniferous kraft pulps, notably high tearing resistance andfolding endurance, high opacity along with high brightness and freedomfrom dirt.

Another important aspect of the invention is the provision of a processthe detailed conditions of which are readily alterable to effect desiredand predictable liber characteristics ranging broadly from thosecommonly associated in the' industry with sulte pulp to those associatedwith kraft pulp. The invention has made possible the production of new,superior papermaking pulps without the necessity of a liquor recoveryprocess which, in

i the case of the kraft process, dictates the minimum economic size ofthe production unit.

Through the present invention there has been provided a process for theproduction of an unusually high tearing strength pulp, equal or superiorto kraft pulp, in which the bleaching is effected by a single, 3-stagesequence instead of the elaborate and costly multi-stage procedurerequired for bleaching kraft pulp.

Yet another advantage resulting from the present invention is theprovision of a process for the production of a bleached high-tearingstrength pulp which can be prepared for the paper machine with much lessexpenditure of power than normally is required in preparing kraft pulpsfor the paper machine, and one in which the amount of power required forthe nal stock preparation may be controlled and reduced during aprevious stage of the pulp production.

Also, the invention provides a process for the effective utilization ofcertain economically desirable woods, such as pines and larches, whichare considered relatively unsuitable for pulping by the sulfite processbecause of inhibiting components or the presence of a substantialproportion of heart wood.

For a proper understanding of the invention, it is necessary first toconsider briefly the two chemical processes by which virtually all thebleached coniferous (longbered) pulp of commerce now is produced.

The conventional sulte pulp is made by cooking wood chips under pressurewith a solution of an alkaline or alkaline earth bisulfite and sulfurdioxide. Chemical removal of lignin and encrusting materials results ina crude pulp which is washed and mechanically screened to rev move knotsand other incompletely defibered material.. The resulting pulp isreadily bleached to yield a relativelyv clean pulp of high brightness.In current practice, the bleaching usually is accomplished in athree-stage sequence of operations involving chlorination,neutralization with dilute caustic soda, and bleaching with calciumhypochlorite. Following each stage the pulp is washed to removeychemicals and solubilized materials.

The sullitc cook is a highly critical operation and the attainment ofuniform results is dependent upon a precise control of all theVariables. Toward the end of a sullite cook, the combined SO2, namelythat equivalent to the amount needed to produce the mono-sulte of themetal present, approaches exhaustion and concurrently the hydrogen ionconcentration in the liquor rapidly increases. Under these conditions,prolongation of the cooking results. in excessive hydrolytic attack onthe cellulose with an accompanying loss in fiber strength. In fact thesultite process finds an important use in supplying dissolving pulps forthe viscose rayon process precisely because of the controlleddegradation of the cellulose, which results4 in readily lterabledispersions. On the other hand, insuflicient cooking results in a rawpulp high in screenings which is diilicult to process in the routinema-nner. It is true that operators become quite skilled in the controlof the variables and normally produce a pulp of reasonable uniformityfor papermaking purposes. At best, however, the result obtained is acompromise. The strength ob tained is considerablyunder the fullpotential of the cellulose bers in the native wood as judged by what canbe obtained by the more selective processes such as the kraft process,and by the present process about to be described.

Usually the sulte process, employing a calcium base liquor, does notinclude a recovery process, in the commonly understood sense ofrecovering the base ion for reuse. However the cook is made in thepresence of a large excess of sulfur dioxide gas and it is necessarytorecover this excess gas. Not only for reasons of economy but toprovide the rich gas required to raise the total SO2 in the raw liquorto a much higher concentration than would be posisble with the lowstrength burner` gas alone. The entire liquor making and gas recoveryvprocess is quite elaborate and critical in its operation.. Briefly itinvolves cooling the burner gas, the counter current absorption of SO2in limestone-packed towers and the refortication of the weak acid thusformedv with the pure relief gas from the cook.

The kraft process, in contrast to the sulte process, employs a stronglyalkaline cooking liquor. Wood chips are digested under pressure with asolution containing, as essential ingredients, sodium hydroxide andsodium sulfide. Since these chemicals are costly, their recovery forreuse is a basic requisite of the process in order to make iteconomically feasible. This recovery involves the Washing of the pulpcounter-currently in order to remove the spent liquor with as littledilution as possible. rthe recovered liquor then is evaporated to such aconcentration of organic matter that it will sustain combustion in therecovery furnace. Prior to combustion, the necessary amount of salt cakeis added to supply the sodium and sulfur which have been lost due to theinefficiency of washing. During the combustion of the black liquor, thesalt cake (sodium sulfate) is reduced to sodium sultide. The smelt fromthe furnacing operation must be causticized with lime in order toconvert the sodium (present in the form of carbonate) to sodiumhydroxide. Many other steps are integral with the recovery process,which cannot be elaborated upon here. The purpose merely is to show thatthe employment of the kraft process involves a very complicated andinitially costly recovery operation in order to salvage the expensivesodium component which necessarily is employed in excess in the, cookingoperation.

While the bleaching of sultite pulp is relatively simple, the fullbleaching of sulfate (or kraft) pulp is a costly and complicatedprocess. The unbleached kraft pulp contains not only residual lignin butother materials which are resistant to the action of the usual bleachingchemicals and consequently are difficult to remove without darnage tothe cellulose. This matter has been the subject of a great deal ofinvestigation. Various workers have attributed the diihculty to thepresence of thio-lignin or to sulfur derivatives of phlobotannin.Whatever may be the reason, only in recent years has it been possible toachieve, through the introduction of new techniques and chemicalshitherto unavailable in commercial quantities, the outstandingly strong,white pulps now on the market. But, whereas the sulfite process and thepresent process require only three stages in the bleaching operation, asmany as 8 or 9 stages frequently are used to bleach high grade kraftpulp. Furthermore, chlorine dioxide is required in one or more of thefinal stages for the bleaching of kraft pulp, in place of theconventional calcium hypochlorite. Chlorine dioxide is too hazardous achemical for shipment and therefore its employment requires theinstallation of a costly plant, at the point of use, for itsmanufacture. Attendant upon the use of chlorine dioxide are hazards bothof a toxic and explosive nature.

As has been shown, the conventional sulfite process, even under the mostfavorable operating conditions, yields a pulp which is unsatisfactoryfor many papermaking applications. Undoubtedly that is an importantreason why the production of bleached kraft pulp has overtaken that ofbleached sullite pulp.

While the kraft process, on the other hand, can yield very strong,highly desirable papermaking pulp, its employment is accompanied bycertain disadvantages. Some of these disadvantages have already beenalluded to. In order to make the operation economically feasible, ahighly complicated, costly recovery process is required. This, togetherwith the necessity of providing a very elaborate multi-stage bleachingsystem adds up to a process which is economically feasible only forlarge production units. Another disadvantage of the kraft process isthat highly noxious odors are developed in the cooking and recoveryoperations making the location of a plant undesirable in an urbancommunity.

As will be appreciated from the foregoing, many integrated sulfitepulping and paper mills have found it necessary, heretofore, to add acertain portion of stronger pulp to their papers to meet strengthrequirements. However, unable, by the size of their operations, tojustify the manufacture of krafbpulp, they have been required topurchase their requirements. This frequently has been found highlyuneconomic and to have numerous other disadvantages of a technicalnature.

In the development of the present invention it has been discovered thatthe advantages of the kraft process can be achieved by a procedure notinvolving its disadvantages. By means of features of the new process apremium quality papermaking pulp can be obtained which exceeds kraftpulp, in tearing strength, and opacity, while equalling it in otherrespects such as folding endurance, brightness and cleanliness.Furthermore, pulp produced in accordance with the invention ischaracterized by an unusually high cellulosic purity and stability.Moreover, these results can be obtained substantially with the equipmentavailable in many pulp mills without recourse to major equipment changesor complicated procedures.

it has been discovered, as an important collateral advantage of the newprocess, that the various details of procedure may be altered at anytime to produce different and predictable fiber characteristics to meetspecific requirements in the paper mill. For example, instead ofproducing at the one extreme a relatively hard beating pulp with a veryhigh tearing strength and high opacity, it is possible, with simplechanges in operation at any time, to produce an easy beating pulp of lowopacity in which the balance of strength properties has been altered tofavor Mullen and tensile strength rather than to emphasize tearingstrength. Such a pulp, it has been found, is highly suitable forapplications in which conventional coniferous sulte has been employed'out is substantially superior to it in all strength factors.Furthermore, within the extremes of characteristics which the conditionsindicated above would produce, it is possible, by the new process, toobtain any desired intermediate characteristics by means of appropriatealterations in process conditions.

The new process can be employed without chemical recovery, thuseliminating the need for a .high production level to justify theinstallation of expensive recovery equipment. -ln this manner the newprocess is economically suited to the production of relatively smallamounts of a selected pulp grade.

An important advantage of the new process is that the liquor-makingfacilties for the preparation of the digestion liquor may be completelyintegrated for the concurrent production of both the sulte and bisulliteof the alkaline metal used-the former supplying the needs of a deciduoussemichemical operation and the latter the needs of the new coniferoussemichemical process. Another highly important advantage of the newprocess is that any of a variety of suitable chemical recovery systemscould be employed for it and equally well in connection with a hardwoodneutral sultite semichemical operation. This makes possible a singlerecovery process in a pulp mill producing a very wide range ofpapermaking pulp, for example: l) very tough high tearing strength pulpssimilar to kraft, (2) easy beating, high tensile strength pulps similarto sullite, and (3) deciduous semichemical pulps with well balancedstrength and good formation characteristics.

Since the usage of soluble base is about the same in the new process asin a conventional acid sullite process employing soluble base, the sameincentive for recovery would apply in both cases. Except to set forthmethods for economical employment of the sodium constituent in stages ofthe process, it is not within the scope of this invention further todiscuss recovery which is not a requisite for the economical productionof the special pulp.

The unique flexibility and superior results obtained with the newprocess are achieved through the novel and essential combination of thefollowing steps:

(l) The control of the degree of cooking througr the applicaiton of anaqueous solution of substantially sodium bisulfite, of a specifiedanalysis and hydrogen ion concentration, in the partial digestion ofconiferous wood chips to give, after suitable mechanical tiberizing, asemichemical pulp having a predetermined permanganate number.

(2) The control of the degree of an alkaline digestion of thesemichemical pulp7 conveniently, but not necessarily, replacing thealkaline neutralization step of the conventional -stage bleach; thechemical concentration and conditions of temperature and time controlledin accordance with predetermined correlations to give a specifiedpercent alkali extractives test on the pulp, which in turn serves as amethod of indicating the physical properties of the pulp.

Coniferous wood chips having a normal length distribution are suitablefor the new process. While chip characteristics are less critical thanwith the acid sultite process, it is preferred that no substantialportion of the chips be less than 1/2 inch nor more than l inch inlength. The chips stored at it (F'tG. l) are charged to a digester l1provided with external heaters and a circulation systern. The digestershould be equipped with a stainless steel lining or a brick lining ofthe type supplied for soluble base acid cooking.

The chips preferably are presteamed for about 30 minutes by injectingsteam in the bottom of the digester with the top relief valve open.ollowing the steaming operation, the chips are covered with the cookingliquor, delivered from a tank 1.2. This liquor is essentialy a solutionof sodium bisulite with a pH within the range of 3.0 to 5.0. There may,however, be a slight excess or deficiency of SO2 present, as comparedwith that required to have the sodium present in the form of bisullite.Expressed in the convention employed in acid sulfite pulping theanalysis, based on the weight of the solution, is

about: Percent Total SO2 1.50-2.40 Free SO2 .7S-1.20 Combined SO2.7S-1.20-

it will be understood that by combined SO2 is meant that required toform the monosulfite, while free SC2 is used to designate that in excessof what is required to form the monosuifite. The liquor to wood ratio tomaintain a coverage of the chips will vary according to their density,and usually should be Within the range of 5 to- 1, to 6 to 1. Thechemical concentration in the liquor is preferably such that theequivalent of between 9.9- and 12.0% total S02 and 3.0 and 4.0% Naz() onwood will be employed.

This example is typical for spruce and balsam fir. Other coniferouswoods may require slightly dilferent con-- centrations and the properconcentration should be established for each particular species. Theactual chemical consumption, as influenced by chemical to wood ratio andcooking conditions, influences important factors such as powerrequirements in iiberizing, pulp yield, pentosan content, chlorinerequirement in bleaching, and strength. As conditions of the coo-kapproach those of complete pulping, strength characteristics and pulpyield are adversely affected. On the other hand conditions resulting inexcessively raw cooks give chips which are dirhcult to resolve byberizing into discrete fibers. his results in Shivy pulp-s which requireexcessive amounts of `chlorine in bleaching. Normally the permanganatenumber for best results will not exceed 60 nor will be less than 30. Formaximum strength development we prefer to voperate as close to the upperlimit as possible although highly desirable results are obtained withinthe entire range cited.

The cooking liquor is simply prepared by passing burner gas into asolution of soda ash of the proper concentration until the desired pH ortotal SO2 is attained. Since no excess SO2 beyond the formation of thebis `te is desired, SC2 from the burner gas is readily absorbed to givethe desired total SO2 with very small absorption towers and without theneed for fortification with concentrated SO2 gas as is the case with theconventional sulite process. Furthermore, it is not necessary to coolthe burner gas preliminary to its injection into the soda ash solution,thus the heat in the gas may be recovered in the solution.

The specific digester operating conditions nre not critical. Theyinvolve an adequate temperature rise period or the use of othertechniques to avoid burning the chips. About 2 to 3 hours is usuallysufhcient.

For most economical operation the chemical to wood ratio is soproportioned that the desired permanganate number has been obtained whenthe total SC2 at the end of the cook is substantially exhausted.

The cooking temperature is not critical but usually will be within therange of i60-170 C. Because the reaction rate is very rapid at 170 C. itis pre erred not to exceed this temperature since the control of thecooking time becomes increasingly critical. it has been found that thechemical requirement is about 16.2% llalel()3 (10% SO2) for spruce wood.However, other conifers will require slightly different concentrationsof chemical and this is best determined by actual trial. When thedesired end Atest (percent total SO2) in reached the digster charge isblown to a blowpit ft2-l usually after the pressure has been relieved toabout 55 psi. The end test is made by titrating a sample with4.potassium iodate according to the method of llalmrose. From the blowpit the spent liquor and wash water are drawn off through a line 1d andthe partially digested and defibered material is delivered to a chiptank l5.

Although a vertical digester has been used in the conduct of thisportion of the process, it will be obvious that the cook could readilybe made in other suitable equipment. For example, a rotary digester orany of the various continuous digesters now available could be used.This is in contrast with the conventional-sulte process which isdependent upon batch cooking because of the presence of excess SO2 inthe cooking liquor. Furthermore, while it is preferred to conduct thebisulite cook in the liquid phase, it is entirely feasible to employvapor phase cooking which is relatively common in a number of millsproducing hardwood semichernical pulp. 1n s1 a variation of technique,the chips are subiected to a perictration period inthe presence of amore concentrated solution of the chemicals and at a temperature ofabout C. Following the penetration period, the excess liquor is drawnoff leaving only a :small amount for basting or circulation purposes.The temperature is brought to maximum and the cook completedsubstantially in the vapor phase.

The `cooked chips discharged to the blowpit are in the form of crudepulp but not substantially deflbered as in the case of acid sulte pulp.On the other hand the cooked chips are much more completely liberizedthan is the case with hardwood `semicnemical pulp of a correspondinglignin content. A relatively mild mechanical treatment is adequate toreduce this material to a cornpletely iiberized form. For this purpose arotary disk mill of the single or double rotating plate variety may beemployed. Any of a great variety of attrition mills which will befamiliar to those in the industry, may likewise perform the functionrequired.

In line with the foregoing, the material collected in tank 1S may all bedelivered through a line 1d to a knetter screen 17' from which the goodfraction, consisting of the iiberized and largely defibered or brokendown material passing through the screen, is delivered by a line 13 to adecker 19 and then to a chest Ztl, from which it is delivered to a discrefiner or the like of the character mentioned. From this reflner thematerial may be passed to a chest 22 and then preferably to acentrifugal cleaner system 23 of the type disclosed in the patent toSamson, et al., No. 2,377,524, granted June 5, 1945'. As explained inthat patent, the centrifugal cleaner system results in the eliminationof shives and other undesired constituents of the pulp. A highpercentage of the pulp passing through the centrifugal cleaner systemconstitutes good pulp which may be sent to a thickener 24 and then tothe pulp purification and bleaching stages of the process schematicallyshown in WEll-G. 2. As shown in FIG. 1, the material collected in chest22 may, if desired, be first subjected to a fiat screening operation, byscreen 22a, before being passed through the centrifugal cleaner. 1n comecases this option may be employed advantageously to remove shives dirtparticles which have no-t been resolved completely in the hberizingoperation.

The material remaining on the knottcr screen i7 contains a certainamount of good pulp and this may be recovered by passing the indicatedfraction to a tank 25 from which it may be delivered to a partialpulping unit 26 of the character to be described more fully hereinafter.The partially pulped material may then be delivered to a knotter screen27 and the good fraction passing through the latter may be deliveredthrough a line 23 to the line 13 for inclusion with the good fractionfrom knotter screen 17. The knots, compression wood and the likeremaining on the knetter screen 2'? are rejected, as indicated at 29 inEEG. l.

it is preferred, however, to conduct the libration in the manner taughtin the patent to Rasch et al., No. 2,847,304.

granted August 12, l958. Present with the cooked chips are knots, grosspieces of wood, compression wood and the like which have resisted thesoftening action of the cook and are substantially harder and moreresistant to mechanical disintegration. This material when mechanicallydisintegrated results in dirt particles and shives which are dilicult toremove from the discrete fibers. In order to produce a pulp of themaximum cleanliness it is desirable to eliminate such dirt-formingmaterial before it has gone through the iibraton treatment. For thispurpose the cooked chips, after suitable washing in the blowpit, are, inthe manner taught in said Rasch et al. patent, subjected to a milddisintegrating action either batchwise in a Dynopulper or a Hydropulper,or continuously in a drubber, or the like, in such a manner that theknots, gross pieces of wood, pieces of compression wood and the like,because of their relative hardness, are not substantially reduced insize while the desirable softened wood chips are reduced to small beraggregates or even to individual fibers. For this purpose the materialcollectM ed in tank 1.5 is rst passed to a partial pulping unit Si), ofthe character mentioned, which may be of the same type as that shown at26. From unit 3i) the entire pulp mass is passed to the knotter screen17 and from this point the treatment is the same as described above.

The openings in the knotter plates of screens i7 and 27 are preferablyabout it to 1/2 inch in diameter. rl`laey should be of such size as toeiifect the most eiicient scparation and rejection of the undesirablefraction of gross pieces from the relatively clean material. The latterpasses through the openings of the knotter plates while the gross,dirt-forming material is tailed ed and rejected. However, that tailedofi' from screen 17 may be sent to a second stage of a milddisintegrating action in the partial pulping unit 26 for furtherrecovery of useful fiber, in accordance with the teaching of said Raschet al. patent.

The large, desirable fraction is usually in excess of 98% of the total(partially deligniiied) cooked chips. It consists of a mixture ofindividual fibers and tiber aggregates. Some mechanical attrition isrequired to reduce these aggregates to discrete fibers, but much lessthan in the parallel case with hardwood semichemical pulps. The acceptedstock from the knetter screens is iibrated preferably in a disk mill,such as reiiner 2i, using a plate setting of about to 5 mils. It will beunderstood, of course, that specific variations must be made forparticular applications. In the iibration relativelv little mechanicaltreatment is required to reduce the ber aggregates to discrete fibers.Normally between about 2 and 5 horsepower days per ton will be adequateto produce the desired tiberizing action.

The power expenditure required for iiberizing naturally will vary inaccordance with the severity of the chemical cook, more drasticallycooked chips requiring less power application than ran/er cooks.available disk mills are suitable whether of the single rotating ordouble rotating type. As with conventional hardwood semichemicalpulping. the results are influenced by the variables of the disk millingoperation, notably by plate design, power application, plate setting,stock consistency, speed of rotors, etc. Normally it is preferred forthe present process to conduct the iiberizing operation in such a mannerthat the chips have been resolved into discrete fibers withoutsubstantial cutting or without excessively reducing the drainagecharacteristics of the pulp. Bv maintaining the Schopper-Rieglerfreeness in excess of 820 ml. the power requirement for producing theabove mentioned fiberizing7 is minimized and subseuuent washing of thepulp following the various stages of bleaching is facilitated. Chemicaleconomy is correspondingly favored. A free pulp, i.e. a free drainingpulp, is normally preferred, but in certain special cases it has beenfound advantageous to apply more power during the flberizing stage thanis necessary for complete ber separation. This results in a certainamount of iiber cutting and fibrillation along with a substantialreduction Any of the commercially f of drainage rate, or freeness. Thuswhile a normally preferred berizing treatment might result in a ZO-meshfiber fraction of remaining on a ZO-mesh screen and a Schepper-Rieglerfreeness of S4() ml., a controlled application of more power duringiiberizing might result in a 2Gmesh ber fraction of 55% remaining onsuch a screen and a freeness of 750 ml. If desired, the fiberizingtreatment may be carried to the point of producing a 700 ml. freeness.The Schopper-Riegler referred to has been determined by Method 414,institute of Paper Chemistry, Oct. l, i951, while the Ztl-mesh fractionhas been determined by Method 415 of said institute. The additionalapplication of power over that required for berizing results in stockpreparation which normally would be doneand in the case of chemicalpulps always is done-on the final bleached stock in its preparation forthe paper machine. The increased power application in iiberizing at thisstage will be saved in power ordinarily required for stock preparation.In the manner indicated it is possible to produce easy beating pulpswhich are highly desirable in cases when it is necessary to minimizestock preparation because of the lack or limitation of necessaryrefining equipment in the paper mill. The use of commercial bleachedkraft pulps requires a relatively large rening or beating capacity andthe application of a substantial amount of power in order to render thefibers suitable for conversion into fine paper grades.

rhe unbleached pulp, prepared in accordance with the procedurepreviously set forth is clearly in the class of semichemical pulps inthat a partial cook resulting in a softening of the chips, and partialor incomplete delignification, is followed by a mechanical iibration ofthe chips.

it has been yfound of no value for the attainment of the objectives ofthis invention to carry out a conventional semichemical cook onconiferous woods followed by normal bleaching. Only when thesemichemical cook is conducted under certain specified conditions,leading to a speciiied degree of delignilication as established by theTAPFI. permanganate number, `followed by controlled carbohydrate removalin an alkaline purification stage conducted under specified conditionswill the unique results of the present invention be obtained. indetermining the permanganate numbers referred to herein, 'rhere has beenused the TAPPI. Standard 'HMM-501 with modifications explained by P. L.Leemhuis in TAPiI., vol. 37, No. l, january 1954, pp. 32-38.

The neutral sulfite semichemical process bas not proved satisfactory forthe pulping of coniferous woods. The excessive amount of chemicalrequired for these more highly-lignied woods has made this particularprocess uneconomic and unattractive. It has been found that thedigestion time for these woods is excessively long, or the cookingtemperature excessively high and the pulp properties are notsufliciently enhanced over the softwood sultite pulp of the trade tojustify the greater costs invoived. For example, it has been found thatusing 32% sodium monosuliite ybased on wood about 13.5 hours at C. wererequired to cook spruce chips to a permanganate number of 60, whereasonly 5 hours were required to cook to a permanganate number of 47 whensodium bisulfite was employed. The sodium base used in the bisulite cookwas only 31% of that used in the monosulte cook, and the sulfur dioxideonly 62%, with the production of a substantially easier bleaching pulp.

in the case of hardwoods, the strength of pulp cooked by the neutralsulte semichemical process is enhanced greatly over that cooked by acidsulite process. Using softwoods, however, only the Mullen or burstingstrength is improved, while important characteristics of tear, foldingendurance, opacity are not significantly increased. in contrast to theneutral sultite semichemical cook on softwood, with its excessiverequirements of chemical and digestion time, a cook made with sodiumbisultite under conditions specified for our process requires only l to2 hours at 170 C. and no more net :sulfur and base than l1 normally isemployed in a soluble base acid suliite cook with excess SO2.

.It has been found that excellent results can be obtained by the newprocess even with some variations in conditions. For example, the ratioof sodium o sultite in the liquor may depart somewhat from theequivalent of sodium bisulte. However, it is preferred to keep theatomic ratio of sodium to sulfur within the limits 0.95 to 1.05 and thepH within 3.0 to 5.0. Similarly, the ratio of the bisulte ion to wood,which determines the degree of cooking, may be varied somewhat withoutseriously affecting the ultimate pulp properties. However the bestresults are obtained when the concentration is such as to give apermanganate number of 30 to 60. Below about 25, which enters the rangeof true chemical pulps, a substantially reduced pulp strength results.Above about 60 the requirement of chlorine for bleaching isunnecessarily high and the final pulp is inclined to be shivy.

While semichemical cooking of coniferous woods with sodium bisultite forthe production of bleached high quality paper pulps is not believed tohave been practiced in the industry, the present invention goes beyondthat broad concept. It involves a process using that feature but undercertain special conditions set forth herein. Such controlled digestionof the original softwood chips, in combination with the particularpurification stage about to be described, leads to the production ot'bleached pulps with the superior strength and other highly desirableproperties to which the present invention is directed. it has beenfound, for example, when sodium bisulite liquors with a pH above 5 orbelow 3 are used even though the cook is carried to the desired extent,the hi est potential strength and other properties cannot be achieved inthe subsequent puriiication treatments.

The unbleached pulp prepared in accordance with the new procedurepreviously set forth may be bleached in a conventional 3-stage sequenceof operations involving chlorination, alkaline neutralization andhypoc'ulorite bleaching as is customary for conventional full chemicalsulte pulps. Substantial advantages will result thereby in the way ofincreased yield and strength as compared with conventional sulite pulpprepared from the same wood. However, the full potential of the presentprocess as a means of varying important characteristics of tearingstrength, opacity, bursting strength, cellulosic purity and the likecannot be attained in the usual bleaching sequence. Whereas in theconventional method of bleaching suliite pulp the conditions of thealkaline stage (chemical concentration and temperature) are such asmerely to solubilize chlorinated lignin and to neutralize residual acid,in the new process the conditions of alkali concentration, temperatureand time are varied in order to induce a controlled removal ofcarbohydrate material. This is necessary to produce the enhancement ofparticular fiber characteristics in accordance with the requirements ofthe paper mill. rThe extent of purification, which influences thesecharacteristics, is dependent upon the amount of caustic soda consumed.ln the conventional practice of bleaching sulite pulpa it is usual toapply about 3% caustic soda for about one hour at about 59 C. When thesemild conditions are employed in the bleaching of semichemical pulpproduced in accordance with the present invention, the results aremarkedly superior to those of conventional Yleached sulite pulps.rl-"hus the yield, bursting strength, folding endurance and tearingstrength all are substantially superior while the opacity is lower. Thepulp is relatively easy beating and is particularly well adapted to themanufacture of high tensile strength translucent-type papers. Or in afurnish it may serve admirably to permit the use of larger proportionsof low-cost hardwood pulps than can be used with normal sultite pulps.

However, by substituting a caustic soda digestion for the neutralizingstep, it is possible to remove carbohydrate material in addition tochlorinated liguin and residual acid. With increasing alkali consumptionthe pulp yield decreases as the more soluble carbohydrates, such aspentosans, are removed but certain papermaking characteristics areprogressively altered. its the consumption of caustic soda is increasedthe teariim strength, opacity, folding endurance and cellulosic pi. tyare increased. The tearing strength, for example, be increasedsubstantially above that obtainable by the kraft process from a givenwood.

The dev-ice of substituting the caustic soda digestion in place of usualalkaline neutralization stage represents a convenient, the least costlyand normally preferred method of obtaining the desired degree ofcarbohydrate removal. it is entirely possible, however, to place theelke ne digestion in a different relation to the bleachirg sequence ofsteps. For example, `the unbleached pulp may be given the appropriatealkaline digestion treatment and then be followed by the conventionalsequence of steps for bleaching, namely: chlorination, alkalineneutralization and hypochlorite bleachinv. Y

When more than about 6.9% caustic soda on dry pulp is consumed in thealkaline digestion the enhancement of any particular property, such astearing strength is usually not sufcient to justify economically theincreased chemical, additional digestion time and accompanying yieldloss. From practical considerations we prefer to keep the consumption ofcaustic soda within the limits of about 15% to 6% depending upon thetype of papermaking fiber desired. These percentages of caustic sodarefer to amounts consumed by completely washed pulp. ln actual practicevariable amounts of hydrochloric acid carried over from the chlorinationstage are present in the pulp b cause of incomplete washinv.Consequently an increment of caustic soda, to be determined underoperating conditions, must be applied for neutralization purposes abovethat required for controlled carbohydrate removal. At ythe lower range,in which there will be only a small amount of carbohydrate removal, itsumces to treat the chlorinated, washed stock under relatively mildconditions such as C. for one hour with sufficient caustic soda toresult in the consumption of about 1.5% based on fiber. This, however,does not lead to the production of the primary advantages of theinvention.

For the purpose .of attaining very high tearing strength, it isnecessary to employ more caustic and more drastic conditions of time andtemperature in order to consume the requisite amount of caustic soda,which approaches 6.0% caustic soda `on pulp. Satisfactory results havebeen obtained with temperatures ranging from C. to 125 C. and with stockconsistencies from 6 to 12%. Because of the prolonge reaction time inthe lower range, it usually is not practical to make the digestion below95 C. While the same consumption of caustic soda is attained n 3Sminutes at l25 C. as in 4 hours at 103 C., tion in the latter case maybe conducted in open vesseis. To enable treatment at l25 C. specialequipment is required in order to reach the corresponding pressureconditions. rlherefore the selection of the temperature for this stageof the process depends upon the equipment available.

The conditions cited above are illustrative of .the practical extremesemployed in the degree of alkaline digestion in the pur ion sequence. ifit is desired to oba papermal'ing fiber of intermediate characteristicsit will be necessary correspondingly to change the amount of causticconsumed and, in this connection, to make the necessary adjustments inthe temperature and duration of the digestion.

The example following, with the data given in rfable l, shows theresul-ts obtained when conditions of the alkaline extraction are soregulated as to provide various consumptions of caustic soda. The parentunbleached pulp was prepa-red in the manner described below.

rEhe softwood chos used contained 80% spruce and 13- 20% balsam tir. Achip charge of 2481 grams (moist Weight) and 1861 grams (oven-dry basis)was placed in `a laboratory autoclave provided with electrical heatingand forced circulation of liquor. A volume of 7252 ml. of sodiumbisuliite liquor containing 2.30% total SO2 and 1.16% combined SO2 wasadded to the chips in the digester. This provided a ratio of total SO2to Wood of 9.0% and a liquor to Wood ratio of 4.23 to 1. After the coverwas bolted on, lthe temperature was raised on a linear schedule to -amaximum of 160 C. in a period of 3 hours. The maximum temperature in thecook was maintained until a residual total of 0.52% SO2 on liquor wasreached. This required 2 hours and 22 minutes. At this time the digesterwas relieved to atmospheric pressure as rapidly as possible (about 5minutes). The initial pH of the liquor Was 4.4 and the final pH of thespent liquor was 3.55. The cooked chips were washed and given a mildselective pulping in a laboratory Dynopulper for 30 minutes. The crudelypulped chips were screened on a f 4-mesh screen. Knots and otherrelatively hard material Which resisted the mild dynopulping action wereretained on the surface of the screen. This material, which constitutedabout 1.8% of the total weight of pulp,'was dis carded. The crude pulpwhich passed through the screen was iiberized in a 12-inchSprout-Waldron disk reiiner at a pulp consistency of 1.5% and ata plateclearance setting of about 1 mil. The pulp was screened through avibratory screen with 7mil slots but the reject material was negligible.The unbleached pulp had a lignin content of 14.6 and a permanganatenumber of 63.2.

1n the foregoing table the solubility of pulp in cold alkali (alkalisolubility) tests were made i accord ance with the procedure of theSwedish Cellulose Central Laboratory Method CCA-8:53. The determinationor pentosans was made in accordance with the Institute of PaperChemistry Method 424, Pentosaus in Pulp, lanuary 1951.

In the examples cited in Table l a substantial excess of caustic sodawas applied over that consumed. This could be reduced by employingsomewhat higher digestion temperatures or by increasing the reactiontime. it is preferred, however, to apply suiicient excess caustic so thefinal pH of the stock prior to Washing is between 10.5 and 11.5. Thecritical factor in the control of the process at this point is theextent of carbohydrate reni oval which in turn is dependent upon theamount of caustic soda consumed. The requisite caustic consumption willbe in excess of that required for reaction with chlorinated lignin andany residual hydrochloric acid carried over from the chlorination stage.A very useful test tor measuring and controlling the degree of alkalinedigestion desired isprovided by determining,7 the solubility of a knownWeight of pulp in an 18.0% sodium hydroxide solution. The solubility ofpulp in cold alkali values undergo progressive changes in magnitudealong with strength and physical characteristics. Thus the test servesconveniently to control the process and to identity the pulp for itsproper employment in paper mill furnishes. 1n determining the effect ofalkali solubility one should take into consideration the species orconditions of Wood raw ina- TABLE 1 terial used and the details of thesemicnernical cook tollowed. NaOH applied, percent 101g 7-18 4-18 gi) 35Further examples of the new process are set forth in percent 100 S2 7?Tables 2 and 3,1Whichdserve to illustrate the relationship, Durationhrs.. 4 0r n W00 W e Ahe fr@ e a Fina] pHV 12.3 12,4 12.1 (i) (2) anyg'lve pu p bet e ni' .deb .6 of {mkhfnu NaOH consumed (referred to pri7,0), 2 cal pulping and the degree of alkaline digestion agtunsfv 5 7 3'0 1' 6 (l) O final. pulp CilalaCllSlCS. This IClaOlShip was based Alkalisniubiiity, percent 6.0 9.9 11.9 8.1 13.0 upon a Series of cooks made ona mixture of spruce and Pentosans, perccnt 4.6 5. 5 6.9 7.1 4.2 r Yieldoerwoud, percent. 42.0 46.3 48.7 42 44 balsam r chips using fordigestion a solution or sodium l 23 bisuliite under conditionspreviously discussed. in Table Opacity 69 I 63 i 62 67 65 2 ve degreesof semicliemical cooking are indicated with .i q I Commemm bleachedkraftpum 45 different degrees of alkaline digestion, Whne in Table a2Commercial bleached sulphite pulp. the charaCterlStlCS 0f the ISultlilgpulp are Shown.

TABLE 2 Degree of semichemcal cook Degree of alkaline digestion CarstlcCaustic Ex. Total SO2 Cooking Unsoda apsoda con- Temp- Duraappliedretention bleached Lignin Perm anplier sinned eratu re tion of on wood,time at; pulp con tent, ganatc based on based on of di digespcrcent 100C yield, percent number bleached bleached gestion tion, H:M percentpulp, pulp, C. hours percent percent Mild digestion A-. 12. 'i554 50. 53. 42 25 3. 0 1. 00 70 1 0 B ll. 58 3231 53. 2 5. 55 35 3. 4 l. 20 70 l0 C. 10. 60 3109 55. 9 8. 20 45 3. 8 1. 70 l 0 Du.-. 9. 63 2:46 58. 5ll. 5S 55 4. 2 1.50 70 1 0 E-. S. 68 2:24 61. 5 15.70 65 4. 6 1. T0 70 l0 Intermediate digestion F 12. 55 3254 50. 5 3. 42 25 5. 8 2. 00 35 2.0G 11. 58 3:31 53. 2 5. 55 35 5. 2 3. 00 B5 2. 0 H. 10. 60 320i) 55. 9 8.20 45 6. 6 3. 05 85 2.0 I 9. 63 2:46 58. 6 11. 5S 55 7. 0 3. l0 S5 2.0J' 8. 68 2:24 61. 5 l5. 70 65 7. 4 3. 20 35 2. 0

Most drastic digestion 12. 55 3:54 50. 5 3. 42 25 8. 5 5. 50 100 4. 011. 58 3:31 53. 2 5. 55 35 8. 9 5. 55 100 4. 0 10. 60 3209 55. 9 S. 2045 9. 3 5. 60 '100 4. 0 9. 63 2246 58. 6 11. 58 55 9. 7 5. 70 100 4. 0S. G8 2224 6l.. 5 15. 70 65 10. 1 5. 80 100 4. 0

TABLE 3 Bleached Pulp Soluhil- Raw l Yield ol ily in Beating materialbleached Pentosan time to Mullen VFearing Opacity costs iny Ex pilp,caustic content, 750 SR., strength, strength, percent excess of percentsoda, percent mins. p.s.i. g. sulte percent pulp,

percent iL..- 46. 10. 4 5. 85 17 70 65 63 1 26...- 47. 4 10. 85 6.0 1772 68 63 3. 0...- 4B. 0 11. 25 6.15 17 75 70 63 5. 3 D.- 48. 4 11. 6 417 77 72 63 8.1 E 4S. 6 11. 9 6.8 17 79 73 63 11.1

F 43, 7 8.1 4. 8 22 65 78 67l 11.6 G 44. 5 8. 7 4, 85 23 68 S0 66 12. 7H. 45.1 9. l fi. 9 24 71 8l 65 14. 9 45. 6 9. 5 1 24 73 S2 64 18. o J46. 2 9. 9 5.6 24 76 S3 63A 21. 4

K- 40.3 5. 3. 85 3'() 61 Q6 72 21.0 L 41. 0 5. 7 3.9 32 62 102 71 23. 7M 41. 5 5. 85 4. 05 33 63 107 70 25. 9 N. 4l. 8 5. 95 4. 25 64 112 7029. 3 O 42. 2 6.0 4. 55 35 65 115 69 33. 5

The ratio of total SO2 to wood and the duration of 25 caustic sodasolution are passed to a pressure tower 38 the cooks were varied to givea series of products` of varying lignin contents. Each product wasfiberized in a single-rotating disk mill to give completely defiberedpulps and have freenesses within the range from about 84@ to 860 SR.Pulps below 30 permanganate number were substantially chemical pulps andrequired no mechanical fiberizing. The pulps were screened on a .007-inch cut screen plate but inthe case of the mechanically tiberized pulpsthe screenings were negligible. Samples of each of the pulps then werechlorinated in accordance with their requirements as indicated by theirpermanganate numbers. The chlorinated pulps, after washing. weresubjected to three different degrees of alkaline digestion in the mannerpreviously discussed and as indicated in Table 3. rfhe digested pulpswere washed and bleached with buffered calcium hypochlorite to a highbrightness. The pulps, finally washed, were analyzed and tested withresults as shown. For each level of alkaline digestion the pulpcharacteristics are proximate functions of the degree of lignin removal,or, for more convenient and rapid determina-tion, the permanganatenumber. The values set ior'th in Table 2 are interpolated at uniform`increments of permanganate number, namely: 25, 35, 45, and 65, byplotting curves from the actual data taken.

rhe tabula-tion illustrates the relationships which may be developed forany species or condition of wood. It is seen that both the degree of thesemichemical cook and the degree of the alkaline digestion determine thecharacteristics of the pulp obtained. inasmuch as these variablesinvolve the employment of different amounts of chemicals yand lead todifferent pulp yields, there results a corresponding difierence in thecost oi producing the different pulps. These costs relative to that ofconventional sulfite from the same wood are given in the last column ofTable 3. Thus it is possible directly to relate the cost of a pulp withparticular characteristics against the particular set of conditionsrequired to produce the pulp.

En FlG. 2 there is schematically shown a system suitable for thebleaching and alkaline digestion steps. The pulp lderived from thethickener 24 of FIG. l is passed to thebrown stock chest 3l of HG. 2.From here it is sent to a chlorine mixer 32 and thence to a chlorinetower 33 wherein adequate time is provided for `the desired chlorinationof `the stock. From the tower 33 the chlorinated pulp is passed to awasher 34- and then to a mixer S5. into this mixer there is introduced acaustic soda solution from a tank 36 and there is also introduced steamfrom a source 3,7.. From the mixer the pulp and ywherein the desiredalkaline digestion step of the process takes place. Steam is introducedinto the tower 3S from the source `37. When the -alkaline digestion hasbeen completed the pulp is passed to a washer 39 and then to ahypochlorite mixer Calcium hypochlo-rite `is introduced into this mixerfrom a tank 41. A certain amount of caustic soda is also introduced froma tank 42 and steam is injected from a source 43. From the mixer 40 thestock is sent to a hypochlorite tower 44 wherein appropriate time isallowed for the final bleaching of the pulp. The bleached pulp is passedto a washer 4S and then delivered to one or another of a series ofstorage towers 46, 47 and 48. Each of these towers is adapted to retainaparticular grade of pulp so that the appropriate one i-s selected toreceive the particular pulp being processed at the time.

Although only three diierent levels of alkaline digestion were employedto develop .the relationship shown in Table 3, it is obvious that any-set of conditions between the mildest `and the most drastic may beapplied to a pulp within a wide range of lignin contents. This offers aunique flexibility of operation and makes it possible, working frompreviously established reationships to produce any one of a wide rangeof pulps of predetermined characteristics and at a predictable cost.

Another controllable variable not covered in Tables 2. and 3, butpreviously referred to, is the matter of the rening of the unbleachedsemichemical pulp. The term refining is used here in its strict sense asthe mechanical process of preparing fibers for the paper machine. itinvolves the increase of the specific surface of the individual fibersby fibrillizing, fiber cutting, and reducing drainage characteristics,-commonly referred to as hydratingf The process is 4distinct from thatof tiberizing which means :the mechanical reduction of cooked chip-s yorfiber agglomerates into discrete fibers without substantial fibercutting or freeness drop.

During the fiberizing operation a range of choices is available. At oneextreme a stock having a freeness in excess of 850 SR., approaching thatof a full chemical pulp, may be produced. This free-draining, easywashing pulp will, after bleaching, require further mechanical refiningtreatment before it is suitable for the paper machine. At the otherextreme, further application of power during the fiberizing operationcan result in simultaneously refining the pulp to a freeness of, say 750S.R. or less. Thisl pulp, Iafter bleaching, may require little or nofurther treatment of a refining nature in the paper mill.

Such an effect 1s shown in Table 4 as `follows:

TABLE 4 Eect of Refining Durmg Fzberzzing Light Heavy refining refiningduring during Iiberizing tiberizing Unbleached pulp: Schepper-Rieglerfreeness,

ml 860 700 Bleached pulp:

Schepper-Riegler freeness, ml 850 775 .20-mesh fraction, percent 84. 570. 6 Beating time, mins. to 750 S.R 18 3 Mullen strength, p.s.i 80 68Tearing strength, g 60 70 Folding endurance, double folds -v 1700 1530Opacity, percent 64. O 69. G Density (Gurley), seconds 260 200 Beatingtesta-The physical properties of pulps such as bursting (Mullen)strength, tearing strength, folding endurance, opacity and density weredetermined on hand sheets prepared from pulp beaten in a LaboratoryValley beater. The hand sheets were prepared on Noble and Wood sheetmaking equipment from pulp beaten to certain Schopper-liieglerfreenesscs as noted. The weight of the test sheets was 2G pounds on a 17inch by 22 inch, 50() sheet basis. References to the TAPPI standardprocedures followed are as follows.

Laboratory the "EGSM-45 Bursting (Mullen) Strength of Paper, T403M-53Internal Tearing Resistance of Paper, T414M-49 Folding Endurance ofPaper: (il, M LT. Folding Endurance), T423M-50 Opacity of Paper,T425M-44 Air Resistance of Paper (Density), T460M-49 Two identicalbisulte semichemical cooks on spruce chips are represented. ln one casethe cooked chips were iiberized in the disk` mill sufficiently only toseparate the individual fibers while doing the minimum of furtherrefining work on them. The freeness of the unbleached pulp was 86d. Onthe other hand, by substantially increasing the amount of refiningaction, the freeness was reduced to 700. After these t ups were bleachedthe freeness relation persisted at 35i) and 775, respectively, while the2li-mesh fiber fractions were respectively 84.5 and 70.6%, indicatingthe fiber cuttiniT in the latter case characteristic of the effect ofrefining or stock preparation. When these pulps were beaten understandard conditions in a Valley beater 1S minutes was required to beatthe free pulp to a 750 freeness Whereas only 3 minutes was required tobeat the pre-refined pulp to the same freeness. A freeness of 750normally represents the dcgree of stock preparation desired for runningon the paper machine. Balanced against the advantage of a Substantialreduction in stock preparation required in the paper mill, prereiiningresults in certain alterations in fiber characteristics. Thus a markedlyhigher level of opacity was obtained with certain strength factors suchas Mullen and fold lowered while tearing strength was somewhatincreased.

The opportunity afforded by the semichemical operation to includerefining in the fiberizing step, ranging in degree from practically noneto such a degree that no further rening of the bleached stock isrequired to prepare it for running lon the paper machine, while anadvantageous feature of the new process, is of less importance inachieving the purposes of the invention than the semichemical digestionof chips with a bisullite liquor followed by controlled removal ofcarbohydrate material in the purification stage. in fact it ispreferred, for most applications of the process, to employ the mildestdegree of fiberizing of semichernical chips consistent with completedisintegration into discrete fibers. Thev free-drain- Processing Pulp(Beater Method),

ing pulp which results is most economically processed and most easilywashed throughout the various stages of purification. Furtherpreparation of the stock then is done entirely in the paper millbeaters, refiners, Jordans and the like where normally the developmentof paper characteristics can most efficiently and satisfactorily beaccomplished.

The results set forth in Tables 2 and 3 should not be considered aslimiting the scope of the invention. For example, in the `developnientof high translucency-type pulps it may be desirable to carry the cookingyield even higher thus retaining more of the pentosans and othercarbohydrate materials which, because of their hydrophilic andeasy-gelatinizing properties contribute translucency, high tensilestrength, stiffness, and reduced porosity to a sheet of paper. In theusual full chemical pulping processes these materials are, in part,hydrolyzed and lost. Thus, the relationships reported in Tables 2 and 3serve merely to illustrate how, by means of preoise, but simple andimmediately applicable manipulations of 2 major variables the processcan be controlled to produce within a known range of characteristics anyone of `a wide variety lof pulps at a predetermined materials cost.Heretofore, it has not been possible to reproduce within a singlepulping system virtually the cntire range of coniferous papermakingpulps running the gamut from those which have in high degree the uniquecharacteristics of sulte pulps to those which reproduce the specialfeatures of kraft pulps.

The variables whose control -make this achievement possible are:

(1)'Extent of semichemical cooking using a bisulite cooking liquormaintained within a specified pH range, and

(2) Extent of alkaline digestion in the purification stage.

The extent of the semichemical cook limits the potential characteristicswhich may be attained in the finished pulp in a manner illustrated inTables 2 and 3.' Thus the extent of cooking is determined primarily bythe amount of bisulfite ion consumed and is measured by the permanganatenumber. As lthe permanganate number is increased the potential formaximum yield, tearing strength, bursting `and pentosan content isincreased. The potential for chemical purity, and opacity is decreased.Raw material cost is somewhat increased.

The extent of alkaline digestion determines whether the final pulp willbe a sullite-like pulp, a kraft-like pulp or some predeterminedcompromise between the two. r`he amount of caustic soda consumed is thedetermining factor' and the effect is measured and controlled by thetest for solubility in 18% caustic soda. As the alkali solubility isreduced the chemical purity, stability, tearing strength, opacity,softness, bulk characteristics and cost are increased. The yield,pentosan content, Mullen strength are decreased.

The unique opportunity to control the degree of refining of theunbleached pulp is a concomitant advantage of the process. Coincidentwith the berizing operation it is possible to pre-beat the pulp andthuscontrol with precision the amount of refining which subsequently wouldbe required in the paper mill as well as emphasize certain specialcharacteristics. This opportunity of oontrolling the drainagecharacteristics of the pulp is not available in conventional chemicalpulping processes.

Two specific examples of the process as adapted for mill production willnow be given.

EXAMPLE 1 The following procedure has been used in a mill adaptation ofthe process to produce a pulp in which the emphasis was placed upon theattainment of relatively high tearing strength and opacity.

Spruce chips of conventional size were employed. Two vertical milldigesters of 7200 cubic feet capacity were Spadaro employed for theseries of digestione. The digesters, lined with brick joined with aspecial urfural-formaldehyde resin designed to resist soluble oase acidsuliite liquors, were equipped with pumps for forced circulation ofliquor and with facilities yboth for direct and indirect heating. Thecooking liquors employed had the following average analysis:

Total SO2 percent-- 2.40 Free SO2 do 1.19 Combined SO2 do 1.21 pH 3.9

The liquor was prepared by introducing sulfur burner gas into a solutionof sodium carbonate of about 2.0% strength until the desired pH wasreached. Tests for total SO2 and pH adequately served to establish thecorrect balance between free and combined SO2.

Prior to the addition of cooking liquor, direct steam was introducedinto the bottom of the digester in order to aid in sweeping out occludedair from the chips. During this period, which consumed about 1/2 hour,the top relief valve was kept open.

The digester then was filled with liquor and shut in (all valvesclosed). Under these conditions 10.5% total SO2 (in the form of sodiumbisulte), based on dry wood, was available. This was the desiredconcentration. For different wood chip densities, which would alter theliquor to wood ratio, or when different concentrations of chemical towood are desired it is necessary to change the liquor concentrationaccordingly. The liquor to wood ratio in this example was about 5.3 to1.

With the automatic relief valve on the digester set for 1,00 p.s.i.,direct and indirect steam were started and the temperature of the chargegradually brought to 160 C. The digester was kept at 160 C. until a testof the liquor had dropped to an average of about 0.70% total SO2. Atthis time, the digester was gradually relieved to 50 p.s.i. About 3hours was taken to reach the maximum temperature of 160C. The time atmaximum tempera- The mixture of cooked chips and crude pulp in theblowpit were drained of excess spent liquor and washed in situ. Theaverage permanganate number of the stock as determined on thesubsequently iiberized pulp was 45.1.

Since the cook was somewhat on the soft side, the explosive elfect ofblowing the chips from the digester amounted to a partial preliminaryiiberizing action as a result of which a c-ertain proportion of thechips were reduced to a crude pulp-like form. The stock was taken over avibratory knotter screen equipped with 1r-inch diameter perforations.This operation resulted in a segregation with the crude pulp and fiberagglomerates passing through the screen while knots, wood chunks,compression wood, oversized chips and the like, along with uniformlycooked chips too large to pass through the screen, were rejected. Theaccepted material was sent to a storage tank for subsequent berizingwhile the knotter rejects containing shive-forming and dirt-formingknots and the like along with a remaining proportion of acceptable ibermaterial was sent to a Dynopulper. It has been found that rawersemichemical cooks with a lignin content in excess of about 12% containa much smaller proportion of fragmentized chips and crude pulp relativeto unreduced chips. For such material the advantage cf a preliminaryknotting will be correspondingly less and it may be preferable tosubject the cooked chips directly to the selective mild pulping actionin a Dynopulper or continuous druhber prior to knotting. The relativeproportion oi fragmentized material will r etermine the most economicprocedure to follow. The reject material was subjected, in theDynopulper, to a mild pulping action for 10 minutes at about 8%consistency in order selectively to disintegrate the desirable fibermaterial while leaving the harder dirt forming knots and the likerelatively intact. The mixture of knots and crude liber then wassubjected to a second screening on the knetter. The accepts from thisoperation were pumped to the storage chest wit-h accepts from the firstscreening operation for subsequent tiberizing. The rejects comprisingdirt-forming and shive-forming knots and the like amounted to less than1% of the original wood charged. Because of their high shive and dirtpotential they were discarded.

The combined accept material was then subjected to iiberizing inconventional-type disk mills such as are widely employed in theproduction of hardwood semichemical pulp. The disk mills were equippedwith plates having a closed periphery and tie clearance and powerapplication were controlled to maintain a SR. freeness of about 800 ml.After such iiberizing the pulp was screened on dat screens with .007slots and after thiclening was sent to the brown stock chest forpurication and bleaching.

Purification and bleaching of the pulp was carried out in a 3-stepprocedure comprising chlorination, a digestion with caustic soda andbleaching with calcium hypochlorite. Chlorine was applied on the basisof, a correlation chart relating the variables of permanganate number ofthe unbleached pulp, percent chlorine applied and the permanganatenumber desired in the chlorinated alkalinedigested pulp. Approximately11.7% chlorine, based on dry weight of pulp, was introduced into thestock slurry at about 3.0%. The retention time in the chlorination towerwas about 11/2 hours and the temperature 27 C. The chlorinated stock waswashed on a drum washer to remove solubilized chlorinated organicmaterials. The stock thickened to about 12% consistency was dropped to amixer into which a caustic soda solution was added and steam injectedand the mixture was then dropped into a tile-lined tower and steamingcontrolled to give a temperature of 92-94 C. The retention time in thetower was about 3 hours. About 9.0% caustic soda was added, based onpulp, and the pH in the caustic washer vat was 11.5.

The alkaline digested pulp, after washing to remove solubilizedchlorinated lignin and carbohydrate material, had a permanganate numberof 1.9. The stock then was bleached to a brightness of 87-88 using about1.0% available chlorine in the form of calcium hypochlorite. The stockconsistency was about 10%, the temperature 38 C. and the retention timein the hypochlorite tower about 5 hours. Suiiicient caustic soda wasadded with the hydrochlorite to maintain the linal pH at 8.5-9.0. Thebleached pulp then was washed to remove reaction products.

EXAMPLE 2 The wood raw material, digestion and iiberizing procedureswere essentially as set forth in Example 1. Following the chlorinationstage the degree of alkaline digestion as compared to that in Example 1was substantially less. About 4.5% caustic soda based on dry pulp wasadded. The consistency was 12% and the temperature C. The retentiontimein the tower was about 2,v hours and the final pH in the vat of thecaustic washer 10.8. The washed pulp, which had a permanganate number of2.1, was then bleached with calcium hypochlorite as set forth in Example1.

Table 5 sets forth results obtained on the two new pulps of Examples 1and 2, on conventional sulfte pulp produced in the same mill from thesame type wood and on 3 high quality commercial bleached kraft pulps. Itwill be noted that the new pulp (Example 1), produced by the use ofrelatively drastic conditions of alkaline digestion to favor tearingstrength, was equal or superior to the commercial kraft pulps in mostrespects while being substantially superior in ease of beating. New pulp(Example 2), produced by the use of mild conditions of alkali digestion,to favor high yield and thus approach the relative economy of sulfltepulp, was substantially superior to sulte pulp in all strength factors.In fact it has been found that this pulp is suiliciently strong tosupplant kraft ber in many applications. The new pulp at the same timewas found to have the easy beating characteristics and relatively lowopacity of sulte pulp.

TABLE 5 Corn- New New mer- Commercial pulp pulp cial Kraft pulps Ex. lEx. 2 sultite pulp Brightness 87. 87.0 86. 5 88. 5 87` 0 86. 5 Alkalisolubility,

percent 8.6 10. 9 13. 0 8. 1 Beating time to 750 S.R. freeness, mins 114. 5 13 38 36 40 Mullen, p s i 69 63 50 G5 55 82 Tear, g- 97 68 50 89100 73 M lillen-l-tea 166 131 100 154 155 155 Folding endurance,

double folds 2, 530 1, 800 850 1, 050 685 2, 040 Opacity, percent---66.1 68 67.0 71. 0 70. 5

While the various aspects of the invention have been described inconsiderable detail in the foregoing, with preferred limits placed uponcertain factors, it will be understood that various changes may be madein the procedure folowed and the specie conditions set forth, within thescope of the invention as defined by the appended claims. Thus in lieuof using sodium bisulfite in the first digestion step, other solublebisultes of an alkali metal or alkaline earth metal or their equivalentmay be used, for example bisultes of potassium, lithium or magnesiumWhatever bisultite is employed, the various conditions set forth hereinshould be observed.

We claim:

1. A semichemical process for producing from coniferous wood by the samesequence of steps any one of a variety of different types of pulp havingdilerent characteristics and suitable for use in the production ofvarious qualities of tine papers, which comprises partially but notcompletely digesting coniferous wood chips with a liquor which issubstantially a solution of a water soluble bisulte of an alkalinemetal, said liquor having a pH between 3.0 and 5.0 and being used insuicient volume to provide from about 9.0 to 12.0% equivalent SO2 basedon the oven dried wood, conducting said digestion at from 155 to 170 C.,terminating such digestion when the treating liquor contains less than0.7% SO2, removing knots and other dirt-forming components from the massof partially digested chfps, the foregoing steps being so conducted asto provide a dirt-free pulp yield of a preselecled value between about50% to 62%, subjecting the substantially dirt-free fraction of said massto mechanical treatment to reduce the same to a pulp of discrete fibers,and subjecting said pulp to a succession of treatments comprisingalkaline digestion and bleaching treatments, said alkaline digestiontreatment being carried out with suiiicient alkali concentration andunder such temperature and time factors as to bring about a consumptionof between 1.5% and 6% of alkali, based upon the dry weight of the pulp,and thus remove substantial amounts of hemicellulose present in saidpulp, the extent of alkaline digestion being so controlled as to bringabout a preselected reduction in the pentosan content and the desiredproperties of the pulp, the dry pulp yield on the basis of the weight ofthe original wood being at a preselected value between 40% and 50% butsuch as to leave at least a 3% pentosan content therein.

2. A semichemical process for producing pulp in accordance with claim lin which the digestion liquor is substantially a solution of sodiumbisulte having a preselected total SO2 content between 1.50 and 2.40%and in which the weight of said liquor used is of a preselected amountbetween 5 and 6 times the dry weight of the chips being digested.

3. A semichemical process for producing pulp in accordance with claim 1in which the bisulte digestion is carried to a point at which thepermanganate number of the digested mass is at a preselected valuebetween 30 and 65.

4. A semichemical process for producing pulp in accordance with claim 1in which the dirt-forming components removed from the partially digestedmass are subjected to a mild mechanical treatment to liberate desirableiibers therefrom but without substantially reducing the size of thedirt-forming constituents, the desirable fibers liberated by saidtreatment being then separated from' the dirt-forming constituents andcombined with said dirt-free fraction that is subjected to the furthertreatments specified.

5. A semichemical process for producing pulp in accordance with claim lin which said mechanical treatment of the dirt-free fraction of thepartially digested mass is of such character as to not only iiberizesaid mass but also to refine the fibers by brillation thereof to such anextent that the S.R. freeness of the pulp is reduced to a preselectedvalue between about 700 and 800 rnl.

6. A semichemical process for producing pulp in accordance with claim lin which the alkaline digestion step is preceded by a chlorine bleachingstep to preselected extent and is followed by a hypochlorite bleachingstep to attain the desired brightness, said alkaline digestion stepbeing such as to remove chlorinated lignins and some of thehemicellulose constituents of the pulp but not severe enough to producesubstantial further digestion of the desirable paper forming componentsof the pulp.

7. A semichemical process for producing pulp in accordance with claim 6in which the chlorine bleaching step involves the application to thepulp of between `l0 and 12% chlorine based upon the weight of the drypulp for about 1.5 hours and the hypochlorite step involves theapplication of calcium hypochlorite under conditions bringing about theuse of between 1.0% and 2.0% available chlorine based on the weight ofthe dry pulp.

8. A semichemical process for producing from coniferous wood pulpcapable of imparting to paper produced therewith substantially theproperties provided by kraft pulp, which comprises digesting coniferouswood chips in a liquor which is substantially a solution of a watersoluble bisultlte of an alkaline metal to partially lbut not completelyremove the lignin from the chips, said liquor having a pH between 3.0and 5.0 and being used in such amount and concentration as to provide apreselected amount between 9.0 and 12.0% total sulfur dioxide applied towood substance, conducting said digestion for a time and at such atemperature as to provide a cooked mass having a preselectedperinanganate number between 30 and 65, removing from the cooked massknots and other dirt and shive-forming constituents, reducing thebalance of the cooked mass to discrete fibers, and subjecting thefiberized mass to an alkaline digestion and a bleaching sequence, thealkaline digestion being carried out with a concentration of causticsoda of between 6% and 8% based on the weight of the dry pulp and atsuch a temperature and for such a time as to bring about a consumptionby the cellulose constituents of the pulp of a quantity of caustic sodaequal to a preselected value between 4.0% and 6.0% of the weight of thedry pulp, said alkaline digestion being so conducted under theconditions specified as to cause a reduction of the alkali solubility ofthe pulp to a prescribed value within the range of 5.0 to 8.0%.

9. A semichemical process for producing from coniferous wood pulp of thecharacter of a bleached sul- -tite pulp, Ahowever substantially enhancedin all the major strength properties, which comprises digestingconiferous wood chips in a liquor which is substantially a solution of awater soluble bisullite of alkaline metal to partially but notcompletely remove the lignin from the chips, said liquor having a pHbetween 3.0 and 5 .0 and being used in such amount and concentration asto provide between 9.0 and 12.0% total sulfur dioxide applied to woodsubstance, conducting said digestion for a time and at such atemperature as to provide a cooked mass having a permanganate number ata preselected value between 30 and 65, removing from the cooked massknots and other dirt and shive-orming constituents, reducing the balanceof the cooked rnass to discrete fibers, and subjecting the iberized massto an alkaline digestion and a bleaching sequence, the alkalinedigestion being carried out with a concentration of caustic soda ofbetween 3% and 4% based on the Weight of the dry pulp and at suchtemperature and for such a time as to bring about a consumption by thecellulose constituents of the pulp of a quantity of caustic soda equalto a preselected value between 1.0% and 2.0% of the weight of the drypulp, said alkaline digestion being so conducted under the conditionsspecified as to cause a reduction of the alkali solubility of the pulpto a preselected value within the range of 11.0 to 14.0%.

10. A semichernical process for producing from conifcrous wood a pulpcapable of imparting to paper produced therewith properties intermediatein character to those of a bleached kraft pulp and a bleached sultepulp, which comprises digesting coniferous wood chips in a vliquor whichis substantially a solution of a water soluble bisullte of an alkalinemetal *to partially but not compe'tely remove the lignin from the chips,said liquor having a pH between 3.0 and l5.0 and being used in suchramount and concentration as to provide a preselected amount between 9.0and 12.0% total sulfur dioxide applied to wood substance, conductingsaid digestion for a time and at such a temperature as to provide aCooke mass vhaving a preselected permanganate number between 30 and 65,removing from the cooked mass knots and other dirt and strive-formingconstituents, reducing the balance of the cooked mass to discretelibers, and subjecting the fiberized mass to an alkaline digestion and ableaching sequence, the alkaline digestion being carried out with aconcentration of caustic soda of between 4% and 6% based on the weightof the dry pulp and at such a temperature and for such a tirne as tobring about a consumption by the cellulose constituents of the pulp of aquantity ot caustic soda equal to a preselected value between 2.0% and4.0% of the weight of the dry pulp, said alkaline digestion being soconducted under the conditions specied as to cause a reduction of thealkali `solubility of the pulp to a preselected point within the rangefrom more than 8.0% to less than 11.0%.

References Cited in the file of this patent UNITED STATES PATENTS1,838,326 Richter Dec. 29, 193i FOREIGN eATENTs 736,3()0 Great Britain 1Sept. '7, 1955 OTHER REFERENCES Casey; Pulp and Paper, Vol. l, publishedby Interscience Publishers, 1960, pp, 12'9, 152, 153, 172. 173'l 341 and368,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Ratent No.3,069,310 December 18, 1962 Royal H. Rasch et al.

It is hereby certified that error appears in the above numbered patl entrequiring correction and that the said Letters Patent should read ascorrected below.

Column 2, line 65, after "good" insert wet column 3, line 33, for"premnum" read premium column 7, line 73, for "in" read is same line373, for "dgster" read digester 4-; column 8, line 57, for "screen" readscreens column 20, line 5l, for "hydrochlorite" read hypochlorite column2l, TABLE 5, second column, opposite "mins", for "l"I read 2l ;A line54, for ="to" read and column 22, line 46, after "wood" insert a line72, for "precribed" read preselected line 75, after "wood" insert aSigned and sealed this 18th day of June 1963.

SEAL) lttest:

IRNEST W. SWIDER DAVID L. LADD lttesting Officer Commissioner of Patents

1. A SEMICHEMICAL PROCESS FOR PRODUCING FROM CONIFEROUS WOOD BY THE SAMESEQUENCE OF STEPS ANY ONE OF